2. Hold down the right mouse button
The error message will appear, Hit Enter. (Without releasing the mouse button, You’re still holding it down)
Once you hit enter, the message will disappear.
Now you can release the right-click and the it’ll work 😉

To extend this tip so it’s even more useful, you can create a bookmark with the JavaScript as the target URL. In firefox, simply right-click the bookmarks bar and “Add Bookmark…”. Give it a name, and paste the JavaScript into “Location:”. Or in Chrome, right-click the bookmarks bar and “Add Page…”.